Hydrological hazards: General floods and flash floods

Definition and characteristics

General floods can be predicted in advance, except in the case of flash floods. The impact of flooding can include destruction of housing, crops, cattle and people. Volunteers assist well in the early days of these situations, helping to move people around and save belongings.

Floods pose specific challenges for emergency response as sometimes vast areas of land are covered with water, making coordination very difficult. Organising logistics, transport and distribution of relief goods is complicated, since the country's infrastructure is often damaged.

Flash floods are sudden and extreme volume of water that flow rapidly and cause inundation. Because of its rapid nature flash floods are difficult to forecast and give people little time to escape or to take food and other essentials with them.

There are usually two phases following flood disasters. During the first phase, people crowd on patches of high, safe ground, together with cattle and other animals, even snakes and scorpions. Drinking water is often very difficult to find and sanitation is terrible.

During the second phase, people start to move back to their homes as soon as possible. This is often a prolonged process (certain areas dry up quicker than others) and once home, people face new challenges including destroyed water systems, wells and bore holes that need cleaning up and disinfecting. A couple of weeks after the flood, high risks for epidemics like cholera, malaria and dengue emerge.

The International Federation response adjusts to meet the needs of each specific circumstance, during the two phases and into rehabilitation. In general response prioritizes rescue and evacuation, temporary shelter materials, safe water and basic sanitation, food supplies, the short term provision of basic health services and the replacement of medical supplies in health facilities.

Mobile clinics can be used successfully, and additional efforts are concentrated on water and sanitation in particular, along with shelter, distribution of emergency food supply, basic health care and field hospitals.

Because homes, transport, tools, livestock and seeds are lost or destroyed in floods and cyclones, help with rebuilding and the restoration of livelihoods will be needed later on.

Subsequent disasters caused by heavy rain and flooding

Dam collapse

May be caused by a shifting of a dam foundation after and earthquake, nearby oil drilling or faulty construction. Earth dams are more likely to collapse when excessive rainfall fills the reservoir to overflowing. The excess water then pours over the top of the dam, gradually washing it down and cutting deep channels into it. This weakens the entire structure so that it then gives way entirely. The result of a dam collapse is a sudden release of large amounts of water which sweep over low lying villages, causing many deaths and injuries.

Downhill sliding or falling movement of cry soil and rock. Landslides are difficult to estimate as an independent phenomenon. It seems appropriate, therefore, to associate landslides with other hazards such as tropical cyclones, severe local storms and river floods. The term landslide is used in its broad sense to include downward and outward movement of slope forming materials (natural rock and soil). It is caused by heavy rain, soil erosion and earth tremors and may also happen in areas under heavy snow.

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